Solid state motor starters are known in the art and are applied to polyphase AC motors in order to provide, among other things, controlled motor current during motor starting. Solid state starters replace the use of conventional electromechanical starters that can provide either across-the-line starting or reduced voltage starting of the motor. While solid state motor starters provide improved control, they produce heat when operated.
Solid state starter comprise a plurality of three-terminal power semiconductor devices, e.g., silicon controlled rectifiers (SCRs), power transistors, gate turn-off devices (GTOs), etc., that are respectively driven into conduction by control signals applied to a control terminal of the device, e.g., the gate terminal of an SCR. In a manner well known in the art, by connecting the semiconductor devices in each AC source phase in series between the source and motor, e.g., as an AC switch, the devices can be selectively driven into conduction during motor starting to regulate the amount of starting current. Specifically, when configured as an AC switch, the semiconductor devices are controlled to conduct during only a portion of each cycle of the power source voltage such that a predetermined maximum current is not exceeded during motor starting. During motor steady state operation, the semiconductor devices are typically controlled to be fully conductive, i.e., resembling a closed switch.
A solid state starter is generally mounted close to its associated motor. For example, in a chiller system, various mechanical components, the motor, and the solid state starter, are mounted on a common structural base. Such systems are referred to hereinafter as skid mounted systems. However, even when the starter is mounted on a common base, the system still requires cabling and cable raceway for interconnecting the starter and motor.
During operation, power semiconductor devices generate heat which must be removed in order to maintain the devices within predetermined operating limits. Specifically, such devices are rated to carry current as a function of device junction cooling capacity so that it is necessary to maintain the junction temperature at or below a maximum value in order to carry a desired amount of current. Power semiconductor devices have been air-cooled or water-cooled. In the case of air-cooled devices, the assembled unit comprising the semiconductor device(s) and associated cooling fins can become physically quite large and therefore require a large physical mounting space. This is especially undesirable in skid mounted equipment where physical compactness of the overall system is usually preferable.
Assembled units comprising water-cooled power semiconductor devices are generally smaller than their air-cooled counterparts. An example of a commercially available water-cooled SCR unit is an open power module as manufactured by Powerex, Inc., of Youngwood, Pa., each such unit including two SCR devices. Use of such units enables allocation of a smaller physical space for their mounting. However, there are problems associated with such units. For example, in an industrial chiller system that provides equipment cooling water, the cooling requirements of the semiconductor devices represent an additional cooling load to the system, if they are cooled by the cooling water. A further disadvantage of utilizing such a primary cooling system is that the cooling water may be at a temperature below the dew point, thereby causing the formation of condensation on or near the semiconductor devices, with commensurate adverse effects on device dielectric strength. Moreover, the direct use of either the water of the chiller system condenser or other cooling system water for semiconductor device cooling can be undesirable because the water may not be sufficiently clean for passage through the cooling elements associated with each semiconductor device. Further, the pressure of the cooling water may be too high for semiconductor device cooling. In addition, commercially available water-cooled SCR units have not provided optimum cooling results during start-up operation when the SCR's produce greater amounts of heat over a short period.
In view of the above problems, others have tried to provide a motor starter which is compact and is liquid-cooled. For example, U.S. Pat. No. 4,084,406 to Brenneman discloses chilling equipment that includes a motor driven compressor and a solid state starter for driving the motor. The SCRs comprising the starter are mounted in a sealed starter box. The disclosed system requires that six electrical terminal studs connected to the SCRs protrude through a cover plate mounted on an outside face of the starter box, the cover plate serving as a cover for the motor terminal box upon being bolted thereto. The motor leads are connected directly to the terminal studs. The starter box in which the SCRs are mounted must be sealed so that it can be filled, during chiller operation, with the refrigerant circulated by the chiller compressor. The circulated refrigerant cools the SCRs. As a result, the SCRs represent a cooling load to the chiller system. If the refrigerant becomes contaminated, it may adversely affect SCR performance. Also, since the refrigerant is circulated by compressor operation, the circulation of cooled refrigerant to fill the starter box may be delayed when the motor is first started. Thus, refrigerant may not be available to cool the SCRs during motor starting when the semiconductor devices carry maximum motor current and generate the greatest heat. Because the semiconductor devices are immersed in a container filled with refrigerant, the gate controls for the semiconductor devices are contained in a separate box.